New guinea pig has bald patch

We picked up two boars from a rescue centre a week ago. The younger one of the pair had a small eye injury, which the lady thought had been caused by scratching following a bath she had given him (she thought it had made his skin dry). We have diligently been putting sudacrem on the injury site, which is now dry and appears to have healed but there now seems to be quite a large bald patch below the eye. He seems happy otherwise, eating and drinking well. I did notice him scratching it a bit today.

I have read that guineas can be prone to mites or fungal infections. Do you think it could be one of these, and if so how do you tell which and what can we treat it with? If it's better to take him to the vet, then we will do but if it's relatively easy to treat then we'd really appreciate any advice.

Took him to the vets today, who said he couldn't really tell what it might be, as the skin itself looks fine and doesn't seem to be irritated. It's quite a small patch so he said to keep a close eye on it and go back in a week or two if it gets any worse. He thought it was possible it was ringworm, but difficult to diagnose at this stage. He gave both Guineas a good check over, which was reassuring.

I think their diet is ok. They have fortified Muesli and I'm giving them a good mix of fruit and veg each day, although it's difficult to tell if they are both getting an equal share!